This invention relates to droplet deposition apparatus and especially to printheads in ink jet printers. In particular it relates to a system for providing liquid ink at proper static (hydraulic) pressure to a series of printheads in a drop-on-demand ink jet printer.
In ink-jet liquid droplet deposition apparatus, it is important that the proper static (hydraulic) pressure, typically a small negative static (hydraulic) pressure, is achieved at each ink jet nozzle to avoid drool of the ink. In commercial ink jet printing applications, it is advantageous to provide a printing apparatus having a large printing area to permit imaging of a large image on a target surface without multiple passes of the surface past the printing head. This may be accomplished by providing a number of liquid droplet deposition apparatus (printheads) vertically stacked or xe2x80x9cstitchedxe2x80x9d together. Vertically stacked liquid droplet deposition apparatus arrangements, however, present special problems associated with the control of static (hydraulic) pressure, and their commercial advantages have heretofore been limited by increasing costs or complexity relating to ink storage and delivery.
In, e.g., WO-A-97 44194 a solution to this problem has been disclosed. In that document an ink-supply system is disclosed for providing ink in an apparatus having a first set of nozzles at a higher elevation than a second set of nozzles, comprising: a first compartment in fluid flow communication with the first set of nozzles, said first compartment being configured to allow the egress of liquid to maintain the level of liquid therein at a desired fill height; a second compartment in fluid flow communication with the second set of nozzles, said second compartment being configured to allow the egress of liquid to maintain the level of liquid in the second compartment at a desired fill height; and a reservoir for supplying liquid to the first and second compartments. A pump circulates the ink. Although the ink supply system disclosed in WO-A-97 44194 does performs well for maintaining the desired level of static (hydraulic) pressure at each liquid droplet deposition apparatus, there is still need, as disclosed in that document, for a degassing device to evacuate the air from the ink that is circulated by a pump. The inclusion of this device makes the construction quite expensive.
The problem of air inclusion in ink supply systems of ink jet printers can be addressed as disclosed in EP-A-916 502. In this disclosure a quite complicated combination of ink tanks, suction pumps and valves are provided to degas the ink and to keep the static (hydraulic) pressure at the nozzles constant. To achieve this latter property, an ink sensor is used for sensing the height of the ink in one of the ink tanks.
It has been recognised that when, circulating ink through an ink supply system to the nozzles of the liquid droplet deposition apparatus, not only inclusion of air is possible but also the inclusion of dust particles. When such particles arrive at the nozzles, they can become attached to the nozzle and eventually block one or more nozzles in the liquid droplet deposition apparatus. This has a detrimental effect on printing quality.
Thus it is still desirable to provide an ink supply system that is simple to construct, that has a small number of moving parts and wherein the static (hydraulic) pressure at the nozzles of the liquid droplet deposition apparatus is kept constant and wherein gas and/or dust inclusion are prevented.
It is an object of the invention to provide an ink supply system for an ink jet printer that is simple to construct, that has a small number of moving parts and wherein the static (hydraulic) pressure at the nozzles of the liquid droplet deposition apparatus is kept constant and wherein gas and/or dust inclusion are prevented.
It is a further object of the invention to provide an ink supply system for a xe2x80x9cpiezoxe2x80x9d ink jet printer wherein the ink supply is circulated and that the flow of ink proceeds with very low or no pulsations so that the ink ejection by exerting xe2x80x9cpiezoxe2x80x9d pressure on the ink (i.e. by applying a pulse to the ink) is not perturbed by pulsations in the ink due to the circulation of the ink.
Further objects and advantages of the invention will become clear from the detailed description herein after.
The objects of the invention are realised by providing an ink jet printer including an ink-supply system comprising:
a lower ink container (100), an upper ink container (104), a liquid droplet deposition apparatus with nozzles (107) wherein an ink circulator (102) is provided for circulating ink from said lower container to said upper container and from said upper container through said liquid droplet deposition apparatus back in to said lower container characterised in that
said upper container, having a bottom wall (110) and side walls (111) and containing ink (101) forming an ink surface (116), has a first compartment (I) with an inlet (114) for ink wherein said inlet contains a flow guiding device (114, 114a) for avoiding directional flow of said ink towards said ink surface and a second compartment (II), separated from said first by a partition (112) placed between said first and second compartments and having an opening so that said two compartments communicate only in the lower half of said upper container.